This invention relates to ink jet printers. More specifically, this invention relates to a novel component for ink jet printers herein called an electrostatic lens for aligning or changing the trajectories of charged droplets emitted at high velocities from a nozzle.
The trajectory of charged ink droplets are difficult to align because the droplets are small, typically from 1 to 25 mils in diameter, and consequently the nozzle orifices are small and difficult to maufacture and assemble. A coarse alignment must be achieved to align the trajectory of droplets emitted by a nozzle with a charging tunnel and a pair of closely spaced deflection plates. The charging tunnel diameter is normally only about from 3 to 10 times the droplet diameter whereas considerable larger spacing separates the deflection plates. Once a coarse alignment is obtained, a vernier or fine alignment is often desired yet difficult to achieve.
The alignment difficulty is compounded in multiple jet printers. For example, in a multi-jet printer as disclosed in U.S. Pat. No. 3,373,437 to Sweet and Cumming, the trajectories of the multiple jets must be aligned relative to each other so as to print a straight row of droplets to match a line of print or pixel positions on a target. Heretofore, electrical techniques have been used to correct for the misalignment of one trajectory relative to another. The target is moved at a constant velocity past the print line. The electrical command to place a droplet at a given pixel position is delayed (or accelerated) a small amount to allow the target to move a distance corresponding to the misalignment of the jet trajectory for that pixel position. Alternately, the charge on the errant droplet is increased (or decreased) to vary its deflection and hence placement on the target. Clearly, the alignment is achieved at the expense of increased complexity to the electrical control circuits for the printer.